Check your mail server, this is not OJS problem. Mail server is sending mails, not OJS. OJS gives messages to mail server only.
You can learn more about DKIM here http://www.dkim.org/
I’m not sure if they could solve this problem, but I had the same one and after a lot of time lost, I finally solved it by creating an “application password” in my gmail account. This type of passwords are exclusive for use in particular applications (like OJS in this case) omitting security problems. The whole problem was that mails weren’t sent because gmail blocked them.